Understanding Peptides and Proteins: Structure, Formation, and Important Peptides, Study notes of Chemistry

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CHM333 LECTURES 8: 9/12/05 FALL 2005 Professor Christine Hrycyna 34 PEPTIDES and PROTEINS - Learned basic chemistry of amino acids – structure and charges - Chemical nature/charges of amino acids is CRUCIAL to the structure and function of proteins - Amino acids can assemble into chains (peptides, polypeptides, proteins) o Can be very short to very long Dipeptide = two amino acids linked Tripeptide = three amino acids linked - Amino acids sometimes called RESIDUES - Identity and function of a protein or peptide is determined by o Amino acid composition o Order of amino acids in the chain o Enormous variety of possible sequences e.g., if you have a protein with 100 aa, there are 1.27 X 10130 possible sequences! - Amino acids are linked by COVALENT BONDS = PEPTIDE BONDS - Peptide bond is an amide linkage formed by a condensation reaction (loss of water) - Brings together the alpha-carboxyl of one amino acid with the alpha-amino of another - Portion of the AA left in the peptide is termed the amino acid RESIDUE o Amino acids sometimes called RESIDUES - R groups remain UNCHANGED – remain active - N-terminal amino and C-terminal carboxyl are also available for further reaction - Reaction is NOT thermodynamically favorable (not spontaneous) o Need energy and other components and instructions to correctly assemble This is the process of protein translation FORMATION OF THE PEPTIDE BOND CHM333 LECTURES 8: 9/12/05 FALL 2005 Professor Christine Hrycyna 35 Animation of peptide bond formation: http://www.cat.cc.md.us/courses/bio141/lecguide/unit4/peptide.html http://www.specialedprep.net/MSAT%20SCIENCE/Cellular%20Biology/Proteins1.htm#HOW - Peptides are always written in the N C direction - Each peptide has ONLY ONE free amino group and ONE free carboxyl group; others are neutralized by formation of the peptide bond IMPORTANT FEATURES OF THE PEPTIDE BOND: 1. Peptide bonds have double bond character resulting from resonance stabilization (C-N bond has 40% double bond character) Animation: http://www.mcphu.edu/netbiochem/movies/peptresn.mov - Results in the peptide bond being PLANAR o Cα1, C, N, Cα2 are all in the same plane - Stronger than a normal bond because of the double bond character - No tendency to protonate - No significant rotation occurs around the peptide bond itself. CHM333 LECTURES 8: 9/12/05 FALL 2005 Professor Christine Hrycyna 38 2. Add in the side chains onto the Cα carbons – the ones that have the CH group – in order left to right. Draw side chains fully protonated as well. 3. Determine if groups are ionized at the pH that you are interested in. Remember if pH > pKa, the H+ is OFF (deprotonated) Let’s say pH 5: - Terminal carboxyl group is DEPROTONATED (5 > 2.21) - Terminal amino group is NOT DEPROTONATED (5 < 9.69) - Carboxyl group on Aspartate in DEPROTONATED (5 > 3.9) Go back to fully protonated figure above and remove 2 protons: One from the terminal carboxyl, and one from the side chain of Asp. Note: In di, tri, or polypeptides, there is only ONE free amino group (N-terminus) and ONE free carboxyl group UNLESS present in R groups. You should be able to write out the full structures of peptides at different pH values. CHM333 LECTURES 8: 9/12/05 FALL 2005 Professor Christine Hrycyna 39 Interesting Peptide in Biological Systems: 1. Glutathione a. Tripeptide of glutamate, cysteine, glycine b. Regulates oxidation/reduction reactions in cells c. Destroys destructive free radicals by scavenging oxidizing agents 2. Oxytocin and Vasopressin a. Pituitary gland peptide hormones b. Oxytocin stimulates uterine contractions during childbirth – induces labor c. Vasopressin stimulates water resorption by kidneys and increases blood pressure (anti-diuretic hormone) 3. Enkephalins and Endorphins a. Brain and nervous system peptides b. Important in control of pain and emotional states 4. Insulin – Peptide hormone that regulates carbohydrate metabolism 5. Synthetic Peptide: Nutrasweet or Aspartame a. L-aspartyl-L-phenylalanine methyl ester b. Dipeptide – 200X sweeter than sucrose and only 1 calorie/teaspoon vs. sucrose that has 16 calories/teaspoon c. Used in diet and low calorie foods and drinks d. Highly profitable e. If amino acids are in the “D” configuration, the peptide is bitter not sweet. CHM333 LECTURES 8: 9/12/05 FALL 2005 Professor Christine Hrycyna 40 Phenylketonuria: - Genetic deficiency in phenylalanine hydroxylase, the enzyme that converts Phe Tyr (autosomal recessive) - Incidence: PKU appears in about 1 in 10,000 births in Caucasian and East Asians. Some are higher: Turks are 1 in 2600, Irish are 1 in 4500, and some are lower - Japanese 1 in 143, 000. Exceedingly rare in Africans. - Accumulation of phenylalanine and its derivatives (e.g. phenylpyruvate is found in high levels in urine) - If left untreated, severe mental retardation occurs after birth due to accumulation of Phe in brain – poisons neurons - Testing of all infants in US and most of Europe began in mid 1960’s - Testing occur about 48 to 72 hours after birth - Treatment: Low Phe diet and blood monitoring - In phenylketonurics, Tyr is an essential amino acid - Reason for warning label on foods containing NutraSweet - One of the most thoroughly tested and studied food additives the FDA has ever approved – considered safe for the general population - Aspartame ingestion does produce methanol, formaldehyde and formate, substances that could be considered toxic, but the levels produced are low. - No “credible evidence” that there is a link between Aspartame and multiple sclerosis, systemic lupus, Alzheimer’s disease or vision problems. - Beware what you read on the internet! Not always factual or credible!